A precise test of the flame-stretch theory of blow-off

Abstract

The postulate that blow-off is caused by excessive flame stretch in the stabilization zone has been tested for methane-air flames inhibited with methyl bromide. This involved the determination of a series of burning velocity vs composition curves corresponding to constant methyl bromide/methane volume ratios of 0, 0.01, 0.02, and 0.03. The maximum burning velocity for methane-air flames was found to be 38.3 cm/sec at 10.25% methane in air. The addition of methyl bromide reduced the maximum burning velocity to 35.1, 32.7, and 30.7 cm/sec for ratios of 0.01, 0.02, and 0.03, respectively, with a shift in the mixture composition at the maximum towards stoichiometric. Blow-off flow rates of inhibited methane-air flames were determined under laminar flow conditions on long cylindrical burners ranging from 0.62 to 1.30 cm diam. For any particular methane-air mixture, blow-off was found to occur at a fixed value of the Karlovitz flame-stretch factor, irrespective of the inhibitor content of the mixture. It is concluded that blow-off is almost certainly caused by excessive flame stretch in the stabilization zone. The critical value of the flame-stretch factor at which blow-off occurs, appears to increase rapidly as the methane content of rich primary mixtures is increased. It is suggested that this apparent increase is due to the error involved in the assumption that the burning velocity in the flame-stabilization zone is the normal burning velocity of the primary mixture. The burning velocity in the stabilization zone at the base of the flame will be significantly affected by (a) intermixing of the primary mixture with the surrounding atmosphere prior to combustion, and (b) heat transfer to the primary flame front from the outer diffusion flame.